magit-commit-{internal,fallback}: clarify separation

What was I thinking when making the original split?!
magit-commit-async: rename from magit-commit-internal
magit-bisect-async: rename from magit-run-git-bisect
magit-rebase-async: new function
magit-run-git-with-editor: new function
magit-diff-while-committing: use correct diff buffer
magit-diff-while-committing: cosmetics